1. Field of the Invention
The present invention relates to hydraulic braking systems and more particularly to an improved vehicle hydraulic brake master cylinder.
2. Description of the Related Art
Vehicle hydraulic braking systems and the master cylinder designs utilized therein are old and well known. In its simplest form, the master cylinder has a fluid reservoir located above a hydraulic chamber which is closed at one end by a spring biased movable piston and opens at the other end into one or more individual brake lines. The master cylinder functions, upon mechanical actuation (often vacuum boosted) of the movable piston to supply brake applying fluid pressure to one or more vehicle wheel braking cylinders. Relatively simple master cylinders of this type are shown, for example, in U.S. Pat. Nos. 4,320,624 and 6,658,844. It is not uncommon to split vehicle hydraulic circuits into two or more nearly autonomous circuits, for example, one circuit might actuate the front wheel brakes while another operates the rear wheel brakes. One way to effect such a split is to employ a single master cylinder having to separate, but simultaneously actuable hydraulic chambers. Separate fluid reservoirs or a single shared reservoir may be employed. Master cylinders having two separate chambers are illustrated in U.S. Pat. Nos. 4,122,596 and 5,513,492 among others. This latter patent illustrates a tandem master cylinder having a cylinder body containing primary and secondary pistons. The primary piston has an integrally formed force input rod through which it is actuated from a driver's pedal or a booster device. The secondary piston is coupled to the primary piston for simultaneous operation by way of springs acting between the pistons with the combined strength of the springs being more than a secondary piston return spring acting between the secondary piston and the blind end of the cylinder body. The secondary piston carries a forward seal which, together with the internal cylinder wall, defines a pressure chamber. A further pressure chamber is defined between the pistons by a seal carried by the primary piston and a further seal carried at the rear end of the secondary piston. The cylinder body provides a pair of reservoir connections which communicate respectively with separate fluid chambers of a fluid reservoir.
The aforementioned U.S. Pat. No. 4,122,596 observes that due to the high pressures involved and limited space, master brake cylinders have been conventionally formed as relatively thick-walled but simple castings by utilizing either sand or metal cores. Whichever method of coring is employed, the critical rubbing surfaces of the master brake cylinder must be machined to eliminate scratches resulting from stripping of coring and to define a better surface against which the piston may bear. Finally, the patentee observes the cost of machining is one of the undesirable aspects of conventionally made brake cylinders. To reduce weight and avoid machining of brake cylinders, the patented arrangement employs an integral composite casting about a seamless steel tube of critical thickness, closed at one end, and treated by acid etching on its exterior. The tube is pretreated to contain critically dimensioned punched and coined openings for brake fluid filling and fluid delivery. The tube is preheated to a predetermined temperature and molten magnesium is cast thereabout producing a compression fit about the tube as a result of solidification shrinkage. Thus, in addition to the traditional casting and bore honing, the prior patented master cylinder forming techniques range from the simple master cylinder and reservoir formed of a single molded plastic piece a shown by U.S. Pat. No. 6,658,444 to this rather complex molding about a cylindrical insert.
Applicant's Assignee currently manufactures a heavy duty (large bore) master cylinder in which such conventionally formed relatively thick-walled but simple castings utilizing either sand or metal cores have not been satisfactory. In order to maintain structural and metallurgical integrity, these master cylinder body castings cannot core the internal bore. The bore is formed by machining the previously cast body. Significant stock removal is therefore required to create the main bore. This process is wasteful both in the casting and increased machining cycle time. Finish wall thickness and fixturing variations further increase the casting OD to insure structural integrity. The increasing cost of aluminum alloy has made manufacture of the big bore master cylinder designs significantly more expensive. An economic analysis of this product has revealed the master cylinder casting to be far the most costly feature of the master cylinder.
It is highly desirable to move away from a massive body casting for large bore master cylinders. This will increase design flexibility as well as minimizing piece part cost and tooling investment.